1 Internet Networking Spring 2006 Tutorial 1 Subnetting and CIDR

2 Administrative Information Course site: webcourse.cs.technion.ac.il/ Assistant: Anna Levin, Taub 323, (829)

3 IP Addressing: Original Classful Scheme IP Address – 32-bit integer globally unique address Dotted Notation: IP Classes – dividing an address to net id and host id The prefix (net id) identifies a network. The suffix (host id) identifies a host on this network.

4 IP Addressing: Original Classful Scheme Class A – 7 bits to net id, 24 bits to host id – Class B – 14 bits to net id, 16 bits to host id – Class C – 21 bits to net id, 8 bits to host id – Class D – for multicasting Class E – reserved for future use (used for private addresses)

5 Weaknesses of Classful Scheme Growth of routing tables in routers Tens of thousands small (class C) networks. Each network must be advertised. Inflexible Lack of a network classes for mid-sized organization (between class B and C). Address space will be eventually exhausted

6 Classless Inter-Domain Routing (CIDR) The concept of network "class" is deprecated. Routing destinations are represented by network and mask pairs. Routing is performed on a longest-match basis Advantages: Flexibility Enabling network aggregation; thereby reducing the size of routing table. Defined in RFC 1519.

7 Subnet Addressing A site has a single IP network address assigned to it, but has two or more physical networks. Different technologies. Limits of technologies. Network congestion. Security consideration. VLAN – separate one physical network into a few logical networks. Administration (e.g. deferent departments in academic institute).

8 Subnet Addressing From outside it looks like a single network Only local routers know about multiple physical networks inside and how to route traffic among them Host ID is divided into a subnet ID and host ID Accepted as a standard at 1985 (RFC 950).

9 Subnet Routing When a router gets a packet, it isolates by Net mask the packet net id address. Each routing entry contain a net mask. Routing is done on a longest-match basis. If the packet is destined to other network then the router sends it to another router. Otherwise the router sends the packet to the appropriate host on its attached networks.

10 Subnetting - Example R. Network /24 H1H1 H2H2 Network /24 H3H3 H4H Rest of the Internet All traffic to /16 A site with two physical networks. Using subnetting, R advertise these networks as a single network (thus, R accepts all traffic for net ) Internal routing is done according to subnet id (i.e. the third octet of the address).

11 Variable-Length Subnetting Motivation: Consider the case when an organization has a few network of different sizes. When we choose the subnet partitioning, we actually define constant number of possible physical subnetworks with maximum number of hosts on them. Difficult to keep small (waist of subnet numbers) and big (the host id needs more bits) sub networks and there could be unnecessary spending of address space. Solution: Variable-Length Subnetting. A subnet partition is selected on a per-network basis.

12 Example – Configuring a Network with Variable-Length Subnetting We have a network with IP /24 We need to support next sub networks: 6 networks with 26 hosts 3 networks with 10 hosts 4 networks with 2 hosts

13 Example – Configuring a Network with Variable-Length Subnetting If we take subnet mask of /27 bits then we can get 8 sub networks of 30 hosts (all 0’s and all 1’s of host addresses are reserved) We need only 6 such sub networks.

14 Example – Configuring a Network with Variable-Length Subnetting The rest 2 sub networks we will partition by subnet mask of /28 bits. We will get 4 sub networks of 14 hosts in each We need only 3 such sub networks.

15 Example – Configuring a Network with Variable-Length Subnetting The rest we will partition by subnet mask of /30 bits. We will get 4 sub networks of 2 hosts in each.

16 Example – Configuring a Network with Variable-Length Subnetting Subnet mask #1 = / Subnet mask #2 = / Subnet mask #3 = /